Within the integrated circuit industry there is a continuing effort to increase integrated circuit speed as well as device density. As a result of these efforts, there is a trend towards using flip chip technology when packaging complex high speed integrated circuits. Flip chip technology is also known as control collapse chip connection (C4) technology. In C4 technology, the integrated circuit die is flipped upside down. This is opposite to how integrated circuits are typically packaged today using wire bond technology. By flipping the integrated circuit die upside down, ball bonds may be used to provide direct electrical connections between the die and the package. Unlike wire bond technology, which only allows bonding along the periphery of the integrated circuit die, C4 technology allows connections to be placed anywhere on the integrated circuit die surface. This leads to a very low inductance power distribution to the integrated circuit which is another major advantage of C4 technology.
As integrated circuit technology has improved, substantially greater functionality has been incorporated into the devices. As integrated circuits have expanded in functionality, the size of the devices have diminished resulting in higher clocking frequencies and increased power consumption. As a consequence, the integrated circuit devices of today generate more heat while possessing smaller surface areas to dissipate the heat. Therefore, it is important to have a high rate of heat transfer from the integrated circuit package to maintain the junction temperatures of the integrated circuit within safe operating limits. Excessive junction temperatures may affect the performance of the circuit and cause permanent degradation of the chip.
In wire bond technology, the chip is attached to the bottom of the package which is usually a heat slug or a heat spreader. The heat slug or spreader provides a thermal interface to attach heatsinks. Since the heat slug and spreader possess a high degree of mechanical strength, a heatsink can be directly attached to either of these devices. The slug or spreader thus protects the die from outside forces that might otherwise damage the die.
In contrast to wire bond technology, a flip chip is attached to a package via a plurality of fragile solder bump interconnections. Since the force needed to hold a heatsink in place is typically 10-20 lbs, the direct attachment of a heatsink to the backside of flip chip may crush the die or cause a solder bump interconnection to crack or break. Although an underfill material may be placed under the die to cushion the solder bumps from the force being applied to them, the direct application of force onto the contact bumps is highly undesirable.
FIG. 1 illustrates a prior art approach to removing heat from the backside of a flip chip. As shown in FIG. 1, a flip chip 102 is mounted onto a ceramic package substrate 104 via a plurality of solder bump interconnections 106. A ceramic cap 108 is attached to the perimeter of the substrate generally by a solder seal 105. The inner height of cap 108 is designed to be slightly greater than the height of die 102. A thermal grease 112 is positioned between the backside of die 102 and cap 108 to provides a heat flow path from the die to the ceramic cap. The thickness of thermal grease 112 is commonly referred to as the "bond line thickness." A finned heatsink 110 is typically attached to the top of cap 108 by solder 114. Because present day integrated circuit devices dissipate considerably more power than those in the past, it is necessary to optimize the heat removal capability of the packages that house these devices. As such, the bond line thickness between the cap and die must be held to an absolute minimum.
Although the use of a cap protects the die from external forces, it limits the package's ability to remove heat from the die by increasing the thermal resistance of the flow path between die 102 and heatsink 110. For heat to escape from the backside surface of die 102 to heatsink 110 it must first pass through thermal grease 112, cap 108, solder 114 and the boundary interfaces of each of these materials. As a result, the thermal performance of the capped package is limiting.
The use of a capped package also requires that the package substrate and cap have matching coefficients of expansion in order to maintain a proper bond line thickness between the backside of the die and the cap. For this reason, the package substrate and cap are almost always made of a ceramic material. The trend in the integrated circuit industry, however, is to move away from the use of ceramic packages since the costs of these packages are relatively high. Ceramic packages are also heavy. Instead, lighter and less expensive plastic packages are now being used to house integrated circuit chips. The use of caps in conjunction with plastic packages introduces several problems. First, since a cap must be thermally conductive, it is nearly impossible to match the coefficient of expansion of the cap with that of a plastic substrate. In addition, plastic substrates are not as flat and rigid as their ceramic counterparts. These problems preclude the application of a cap to a plastic package since the bond line thickness between the cap and the backside of a die cannot be properly controlled to within the strict tolerances required by today's high powered circuits.
Therefore, what is needed is an apparatus for removing heat from the backside of a packaged flip chip.